Pavement markings, such as those on the centerline and edgeline of the roadway are important in order to provide visual guidance for motor vehicle drivers. It is important for a driver to be able to see the pavement markings from a distance as well as from up close.
Currently, pavement markings typically rely on an optical system comprising transparent microspheres partially embedded in a binder layer containing diffusely reflective pigment particles such as those of titanium dioxide (TiO.sub.2) or lead chromate (PbCrO.sub.4) as reflectors. The microspheres are dropped onto a liquid coating that contains the diffusely reflective pigment. In use, light from the headlamp of a vehicle enters the microsphere and is refracted below to fall on a diffusely reflective pigment. Some portion of the light is returned generally along the original entrance path in the direction of the vehicle so as to be visible to the driver. (see FIG. 1)
The use of a mirror type (specular) reflector, e.g., a layer of aluminum or silver, to improve efficiency as long been understood but is practical only when orientation of the reflector-coated microsphere and light source is possible, that is, when the incident light strikes the microsphere near normal, as with a vertical sign. (see FIG. 2) In almost all commercial retroreflective constructions having a sheet form the specularly reflective material is a metal, i.e., a vacuum-deposited layer of specularly reflective aluminum arranged behind the microsphere, as taught in U.S. Pat. No. 3,005,382 (Weber) or U.S. Pat. No. 3,190,178 (McKenzie), or spaced from the microspheres by a spacing layer as taught in U.S. Pat. No. 2,407,680 (Palmquist et al.). Alternatively, specularly reflecting metal flakes can be contained in the binder, as disclosed in U.S. Pat. No. 2,567,233 (Palmquist et al.).
U.S. Pat. No. 3,418,896 (Rideout) teaches the use of retroreflective elements comprised of glass spheres applied to a pigmented plastic rod which is then chopped into segments. The segments are dropped onto a paint to form a pavement marking. The paint onto which the segments are dropped and into which the segments become embedded may be pigmented for example with titanium dioxide.
Currently available pavement markings work effectively for only a limited range of distances and entrance angles. For example, flat pavement markings, which rely on diffuse pigments, are most easily seen at distances of approximately 80 meters and less. At distances greater than this, the optical efficiency drops off due to the relatively larger entrance angles of the incident light. Patterned pavement markings with vertical components which utilize specular pigments are more effective at the longer distances, acting more like a vertical sign.
The present invention provides a retroreflective article that is useful over a wide range of distances and entrance angles (see FIG. 3) regardless of the orientation of the reflector.